I feel the frustration of users who are having connection issues. I do want to say that there must be something else in the mix (old settings from an upgrade or something like that) because with a relatively new iMac running the latest Sierra I can connect to both H9s and it appears solid.
Often I’ll have to toggle the H9 bluetooth off and then on again (pressing the XYZ buttons simultaneously) before connecting, which seems to resolve most of my issues.
My iPhone and old iPad mini seem to connect easily as well.
I’m not trying to dismiss the issues those above are facing, just add that for some reason I’m not having those issues with my various Apple gear.
I believe I read somewhere the Bluetooth spec differs on older/newer Macs. Are you using 2017 Apple devices to pair multiple H9’s via H9 Control? I wonder if its this spec that might be somehow contributing to this Bluetooth pairing issue.